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NEUROPHARMACOLOGY

In Vivo Pharmacological Characterization of Indiplon, a Novel Pyrazolopyrimidine Sedative-Hypnotic

Departments of Neuroscience (A.C.F., M.A.P., M.J.C., D.L., M.J., K.R.G.), Development (T.K.C., H.P.B.), and Medicinal Chemistry (R.S.G.), Neurocrine Biosciences Inc., San Diego, California

Indiplon (NBI 34060; N-methyl-N-[3-[3-(2-thienylcarbonyl)-pyrazolo[1,5-]pyrimidin-7-yl]phenyl]acetamide), a novel pyrazolopyrimidine and high-affinity allosteric potentiator of GABA_A receptor function, was profiled for its effects in rodents after oral administration. In mice, indiplon inhibited locomotor activity (ED₅₀ = 2.7 mg/kg p.o.) at doses lower than the nonbenzodiazepine hypnotics zolpidem (ED₅₀ = 6.1 mg/kg p.o.) and zaleplon (ED₅₀ = 24.6 mg/kg p.o.), a sedative effect that was reversed by the benzodiazepine site antagonist flumazenil. Indiplon inhibited retention in the mouse passive avoidance paradigm over a dose range and with a temporal profile that coincided with its sedative activity. Indiplon, zolpidem, and zaleplon were equally effective in inhibiting locomotor activity in the rat and produced dose-related deficits on the rotarod. In a rat vigilance paradigm, indiplon, zolpidem, and zaleplon produced performance deficits over a dose range consistent with their sedative effects, although indiplon alone showed no significant increase in response latency. Indiplon produced a small deficit in the delayed nonmatch to sample paradigm at a dose where sedative effects became apparent. Indiplon was active in the rat Vogel test of anxiety, but it showed only a sedative profile in the mouse open field test. The pharmacokinetic profile of indiplon in both rat and mouse was consistent with its pharmacodynamic properties and indicated a rapid T_max, short t_1/2, and excellent blood-brain barrier penetration. Therefore, indiplon has the in vivo profile of an efficacious sedative-hypnotic, in agreement with its in vitro receptor pharmacology as a high-affinity allosteric potentiator of GABA_A receptor function, with selectivity for 1 subunit-containing GABA_A receptors.

Address correspondence to: Dr. Alan C. Foster, Department of Neuroscience, Neurocrine Biosciences, 12790 El Camino Real, San Diego, CA 92130. E-mail: afoster{at}neurocrine.com

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